Di-cation ether salts have been prepared by a method described by Stang et al in a reference cited below. As pointed out by the authors, the di-cation ethers are the first known bis(carbenium ions). The Stang et al method for forming the ethers comprises reaction of non-enolizable activated ketones with trifluoromethanesulfonic ("triflic") anhydride. The cost of triflic anhydride is substantial; therefore, its use is not preferred in large-scale commercial synthesis.
Another method of producing di-cation ethers comprises the alkylation of an ether such as 2,2'-bis(pyridyl) ether. Such ethers can be prepared by the silver salt method described in DeVilliers et al, Rec. Trav. Chim., 76, 647, 1957. The use of silver entails significant expense. Moreover, the method of DeVilliers et al does not form 2,2'-bis(pyridyl) ethers in high yield. In other words, when the method of DeVilliers et al is employed, the desired 2,2'-bis(pyridyl) ether is produced with an undesirable amount of a pyridone such as N-(2'-pyridyl)-2-pyridone: ##STR1## Consequently, the DeVilliers method does not lend itself to a commercially viable reaction sequence for the production of 2,2'-oxydipyridinium salts.
When the silver salt of the Villiers process is replaced by the analogous sodium salt, the pyridone is produced in even greater excess over the bis(pyridyl) ether. Thus, the bis(pyridyl) ethers are not preferred for large-scale commercial production.
From the work of Hopkins et al on the alkylation of 2-hydroxypyridine (paper cited below), it is known that the product composition is very dependent on the nature of the alkylating agent, the nature of the acid acceptor, and the solvent. Generally, it is known to employ the silver salt to obtain O-alkylation, and that even then some N-alkylation is observed. As indicated above, use of silver salts is not preferred for commercial preparation.
Applicants' method for the synthesis of di-cation ethers comprises the reaction of a 1-alkyl-2-halopyridinium salt with 2-hydroxypyridine in the presence of a tertiary organic base. It will be apparent to the skilled practitioner that with such reactants there is a possibility of reaction at the nitrogen as well as at the oxygen of the hydroxypyridine anion. Furthermore Applicants do not employ a silver salt as suggested by the art. Consequently it was therefore surprising that Applicants' method gives good yields of a desired oxygen-reacted product (which can be converted into a 2,2'-oxydipyridinium salt). The preparation of di-cation ethers and their use as hardeners is disclosed in Chen et al application Ser. No. 238,665, filed Aug. 31, 1988, and now allowed.